The optimal energy intake for growing premature infants has not been determined. Moreover, little information is available concerning the influence of energy and protein intake on the rate and composition of weight gain. The composition of weight gain of growing premature infants has previously been examined, albeit indirectly. These infants have been shown to gain fat much faster than the fetus of the same post-conceptional age. This study will demonstrate whether altering the energy intake with a high protein intake influences the composition of weight gain, and whether a higher protein-to-energy ratio in the diet produces growth that is more like that of the fetus, i.e. less fat. The first aim is to compare the utilization of energy fed at two levels to infants with birth weight below 1.5 kg. Two groups of infants are fed equal volumes of two formulas with the same concentrations of protein (2.3 g/dl) and minerals (0.5 g/dl) but different energy densities (72.5 vs 84.5 kcal/dl). When fed 150 ml/kg/d both groups receive 3.5 g protein/kg/d, and the gross energy intakes are 109 and 127 kcal/kg/d. Body weight, length, head circumference, and skinfold thickness are measured serially. Energy intake and excretion (bomb calorimetry), energy expenditure (indirect calorimetry), total body water (deuterium oxide space), and extracellular water (bromide space) are measured twice, when growth is beginning and again after 3 weeks on the experimental formula. By measuring these changes concurrently, it is possible to determine the effect of this isolated difference in energy intake (and protein-energy ratio) on the rate and composition of weight gain and the energy cost of growth. The hypothesis that higher energy intake produces more rapid fat accumulation but no advantage in the accretion of fat-free body mass will be tested. The second aim is to compare the utilization of dietary energy before and during growth. This will allow determination of both components of the energy cost of growth. The energy stored in new tissues will be determined as the portion of the ingested energy that is neither excreted in the urine or feces nor expended for metabolism. The energy expended for synthesis of new tissues will be derived from the regression of energy expenditure versus weight gain. The sum of stored energy and energy expended for synthesis is the total energy cost of growth. This study will provide new information on the utilization of energy and protein by growing premature infants and may help to refine recommended levels of energy and protein intake for these infants.